Population Excess Fraction of Ectopic Pregnancy Due to Chlamydia trachomatis in Finland.

Rantsi et al. present results from a large high-quality prospective population-based case-control study nested within the Finnish maternity cohort. In the largest ever study of its kind, they examine the association between Chlamydia trachomatis (CT) major outer-membrane protein peptide-specific IgG antibodies and ectopic pregnancy (EP), miscarriage, and preterm delivery. Cases for EP and miscarriage were identified through the Hospital Discharge Register from 1998 to 2005 and preterm delivery cases from the FinnishMedical Birth Register from 1988 to 2005. Controls werematched to cases by sampling time, age at serum sampling, and postal code district. Most participants were aged 20 to 34 years. They found no evidence of an association between antichlamydial IgG antibody and miscarriage or preterm birth; however, 21.0% of EP cases and 14.6% of controls were positive giving an odds ratio (OR) in the EP cases vs. matched control group of 1.56 (95% confidence interval, 1.20–2.03). Although this provides good statistical evidence of an association, the result is difficult to interpret in terms of population burden of disease caused by CT infection. This article investigates what this result might mean for the population excess fraction (PEF) of EP due to CT. The PEF is the proportional reduction in disease risk that would be achieved by eliminating the exposure of interest from the population, assuming the exposure is causally related to the disease. A number of formulae have been derived by which the PEF can be estimated from epidemiological data. The formula giving an estimate from case-control studies is